Normally, in a motorcycle, only a rear wheel is driven via a chain or the like from an output shaft of an engine. Thus, a front wheel together with the rear wheel are propelled for traveling on rough-terrain or snow in the related art.
An all-wheel-drive motorcycle in which a rear wheel is driven by an engine and a front wheel is driven by a hydraulic motor is proposed, for example, in JP-A-2000-229596, see FIG. 10.
FIG. 10 of JP-A-2000-229596 is a cross-sectional view of a front wheel hub 41, note that the following reference numerals are used in JP-A-2000-229596, that includes a boss 41b for penetrating an axle formed integrally at an axial center portion of a bottom portion 41a with a double-row of ball bearings 61 for rotatably supporting the boss 41b. An internal gear 64 is fixed to a boundary section between the bottom portion 41a and a cylindrical portion 41c of the hub 41. An output gear 65 (hereinafter referred to as the drive gear 65) of a hydraulic motor 7 is engaged with the internal gear 64 (hereinafter referred to as the driven gear 64) so that the hub 41 is driven by the hydraulic motor 7.
Since the driven gear 64 is provided on the inner periphery of the hub 41, the diameter of the driven gear 64 can only be set to a diameter larger than that of the drive gear 65. Thus, the driven gear 64 has little flexibility in setting the diameter thereof.